$ \left(\dfrac{49}{81}\right)^{-\frac{1}{2}}$
Explanation: $= \left(\dfrac{81}{49}\right)^{\frac{1}{2}}$ Figure out what goes in the blank: $\Big(? \Big)^{2}=\dfrac{81}{49}$ Figure out what goes in the blank: $\Big({\dfrac{9}{7}}\Big)^{2}=\dfrac{81}{49}$ So $\left(\dfrac{49}{81}\right)^{-\frac{1}{2}}=\left(\dfrac{81}{49}\right)^{\frac{1}{2}}=\dfrac{9}{7}$